Vehicle steering system having a rear steering control mechanism

ABSTRACT

A vehicle steering system includes a steering input device, a front steering subsystem, a rear steering subsystem, a first force transmission route, a steering control mechanism, a second force transmission route, and a third force transmission route. The front steering subsystem is coupled to front motive members to steer the front motive members based upon movement of a front steering subsystem input shaft. The rear steering subsystem is coupled to rear motive members to steer the rear motive members based upon movement of a rear steering subsystem input shaft. The first force transmission route extends from the steering input device to the front steering subsystem input shaft, wherein force is transmitted from the input device to the front steering subsystem input shaft to steer the front motive members. The steering control mechanism has a movable input member and a movable output member. The movable input member is movable through a first distance without transmitting force to the output member and is movable through a second distance in which force is transmitted to the output member to move the output member. The second force transmission route extends from the steering input device to the input member, wherein force is transmitted from the input device to the input member to move the input member. The third force transmission route extends from the output member to the rear steering subsystem input shaft to move the rear steering subsystem input shaft and to steer the rear motive members.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present Application is a divisional of U.S. patent application Ser.No. 10/456,376, filed Jun. 6, 2003, which issued as U.S. Pat. No.7,073,620 on Jul. 11, 2006, which is incorporated herein by reference inits entirety.

FIELD OF THE INVENTION

The present invention relates to vehicle steering systems. Inparticular, the present invention relates to vehicles having steerablefront and rear motive members or wheels. More specifically, the presentinvention relates to mechanisms for controlling steering of rear motivemembers.

BACKGROUND OF THE INVENTION

Many vehicles today, especially heavy-duty trucks, tractors and supportvehicles, are provided with all-wheel steering systems for improvedmaneuverability. All-wheel steering systems enable both the front axleand the rear axle of a vehicle to be steered in response to turning of asteering wheel. In one known mechanical all-wheel steering system, therear axle and the front axle are both mechanically linked to thesteering wheel such that rotation of the steering wheel results insimultaneous steering of both axles. In another known electronicsteering system, the rear axle (and wheels) are steered independent ofthe front axles based upon sensed information regarding the front wheeland rear wheel angles. Although an improvement over known mechanicalall-wheel steering systems, such electronic all-wheel steering systemsare more expensive and sometimes unreliable due to the required complexelectronic controls and sensors.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a vehicle steeringsystem includes a steering input device, a front steering subsystem, arear steering subsystem, a first force transmission route, a steeringcontrol mechanism, a second force transmission route, and a third forcetransmission route. The front steering subsystem is coupled to frontmotive members to steer the front motive members based upon movement ofa front steering subsystem input shaft. The rear steering subsystem iscoupled to rear motive members to steer the rear motive members basedupon movement of a rear steering subsystem input shaft. The first forcetransmission route extends from the steering input device to the frontsteering subsystem input shaft, wherein force is transmitted from theinput device to the front steering subsystem input shaft to steer thefront motive members. The steering control mechanism has a movable inputmember and a movable output member. The movable input member is movablethrough a first distance without transmitting force to the output memberand is movable through a second distance in which force is transmittedto the output member to move the output member. The second forcetransmission route extends from the steering input device to the inputmember, wherein force is transmitted from the input device to the inputmember to move the input member. The third force transmission routeextends from the output member to the rear steering subsystem inputshaft to move the rear steering subsystem input shaft and to steer therear motive members.

According to another aspect of the present invention, a vehicle steeringsystem includes a steering input device, a torque splitting device, afront steering subsystem, and a rear steering control mechanism. Thetorque splitting device is coupled to the steering input device and hasa first output and a second output. The front steering subsystem iscoupled to the first output of the torque splitting device and isconfigured to adjust steering of front vehicle motive members. The rearsteering control mechanism includes a movable input member and a movableoutput member. The movable input member is coupled to the second outputof the torque splitting device. The movable output member is coupled toa rear steering subsystem. The rear steering subsystem is configured toadjust steering of rear vehicle motive members. The control mechanismoperates in a rear steering state in which force is transmitted from theinput member to the output member to move the output member and a dwellstate in which the output member does not move in response to movementof the input member.

According to another aspect of the present invention, a method isprovided for controlling a front steering subsystem to steer frontmotive members and a rear steering subsystem to steer rear motivemembers on a vehicle. The method includes the steps of applying a firstforce to a steering input device to move a portion of the device andsimultaneously transmitting a second force and a third force. The secondforce is applied to an input shaft of the front steering subsystem,whereby the front steering subsystem adjusts steering of the frontmotive members based on movement of the input shaft. The third force isapplied to an input member so as to move the input member through afirst distance during which the input member moves relative to an outputmember coupled to an input shaft of the rear steering subsystem andthrough a second distance during which the third force is transmittedfrom the input member to the output member to move the output member andthe input shaft of the rear steering subsystem, whereby the rearsteering subsystem adjusts steering of the rear motive members basedupon movement of the output member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a vehicle including a first embodiment ofa vehicle steering system.

FIG. 2 is a plan view of one preferred embodiment of the vehicle shownin FIG. 1 illustrating a preferred embodiment of the steering systemsupported on a frame.

FIG. 3 is a top plan view of the vehicle of FIG. 2.

FIG. 4 is a sectional view of a rear steering control mechanism of thevehicle shown in FIGS. 2 and 3, illustrating the mechanism in acentering position.

FIG. 5 is a perspective view of an input member and an output member ofthe rear steering control mechanism of FIG. 4.

FIG. 6 is a left end elevational view of the input member and the outputmember shown in FIG. 5.

FIG. 7 illustrates the rear steering control mechanism of FIG. 4 in afirst non-centering position.

FIG. 8 illustrates the rear steering control mechanism of FIG. 4 in asecond non-centering position.

FIG. 9 is a sectional view of a first alternative embodiment of the rearsteering control mechanism of FIG. 4.

FIG. 10 is a sectional view of the mechanism of FIG. 9 taken along line10-10.

FIG. 11 is a sectional view of a second alternative embodiment of therear steering control mechanism of FIG. 4.

FIG. 12 is a sectional view of a third alternative embodiment of therear steering control mechanism of FIG. 4.

FIG. 13 is a sectional view of a fourth alternative embodiment of therear steering control mechanism of FIG. 4.

FIG. 14 is a fragmentary perspective view of a fifth alternativeembodiment of the rear steering control mechanism of FIG. 4.

FIG. 15 is a schematic illustration of a vehicle including a secondalternative embodiment of the steering system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates a vehicle 10 including a vehiclesteering system 12. In addition to steering system 12, vehicle 10additionally includes frame 14 and means for powering or drivingcomponents or motive members of vehicle 10 or powered devices orexternal attachments coupled to vehicle 10. Frame 14 generally supportsthe remaining components or structures of vehicle 10. Frame 14 may haveany one of a variety of conventionally known or future developedconfigurations depending upon the functions of vehicle 10. For example,in some applications, portions of frame 14 may articulate relative toother portions of frame 14.

Vehicle steering system 12 generally includes front motive members 16,rear motive members 18, steering input device 20, front steeringsubsystem 22, rear steering subsystem 24, ratio adjusting device 26, andrear steering control mechanism 28. Front motive members 16 and rearmotive members 18 generally comprise ground motive members configured topropel or move vehicle 10. In one particular embodiment, motive members16 and 18 comprise wheels coupled to axles (not shown). In alternativeembodiments, motive members 16 and 18 may comprise other conventionallyknown or future developed members configured for engaging a ground,track or other surface so as to propel or suspend vehicle 10. Forexample, in one embodiment, motive members 16 and 18 may comprisemovable tracks such as commonly employed on tanks and some tractors.Although motive members 16 and 18 are illustrated as being similar toone another, motive members 16 may alternatively be differentlyconfigured than motive members 18. For example, in one embodiment,motive members 16 may comprise wheels while motive members 18 comprisetracks.

According to one exemplary embodiment in which motive members 18comprise wheels, motive members 18 additionally include a central tireinflation system. A central tire inflation system can be used toincrease or decrease tire pressure based upon an operator input, sensorfeedback, or a combination thereof. A variety of central tire inflationsystems are known to those skilled in the art. One example of a centraltire inflation system is the DANA® SPICER® Central Tire Inflation Systemcommercially available from Eaton Corporation of Cleveland, Ohio.

As further shown by FIG. 1, rear motive members 18 includeconventionally known or future developed wheel end speed ratio reductionfinal drive mechanisms 70 which are coupled to a power source or primemover 72 by a transmission 74. Prime mover 72 generates power to driverear motive members 18. Prime mover 72 comprises any source ofrotational mechanical energy which is derived from a stored energysource such as a liquid or gaseous fuel. Examples are an internalcombustion gas powered engine, a diesel engine, turbines, fuel celldriven motors, an electric motor or any type of motor capable ofproviding rotational mechanical energy to drive rear motive members 18as well as to potentially drive front motive members 16.

Transmission 74 transmits the power from power source 72 to final drivemechanisms 70. In one particular embodiment, transmission 74 comprises aconventionally known or future developed mechanical transmission. Inanother embodiment, transmission 74 comprises a conventionally known orfuture developed hydrostatic transmission. In still other embodiments,transmission 74 comprises a combination hydromechanical transmission. Inother embodiments, transmission 74 may comprise a hybrid transmissionsuch as disclosed in co-pending U.S. patent application Ser. No.10/137,585 entitled HYBRID VEHICLE WITH COMBUSTION ENGINE/ELECTRIC MOTORDRIVE filed on May 2, 2002 by Jon J. Morrow and Christopher K. Yakes,the full disclosure of which is hereby incorporated by reference.

Final drive mechanisms 70 are generally situated within or as part ofrear motive members 18 and are configured to reduce the speed ratio atthe far end of transmission 74. For example, in one application whereshafts extend from opposite sides of a differential to wheels comprisingrear motive members 18, such shafts may be rotated at a first speed.Drive mechanisms 70 reduce the speed to increase the torque which isgenerally required for heavy duty wheel and track-type vehicles. Becausedrive mechanisms 70 are located in or formed as part of rear motivemembers 18, the torques required to be transmitted by the shaftsextending from the differential are substantially reduced, enabling suchshafts to be smaller and lighter in weight.

In one embodiment, final drive mechanisms 70 include a multi-stageplanetary mechanism within the rotating housing of rear motive members18. For example, two juxtaposed and interconnected simple planetary gearsets or stages may be used to provide the desired speed ratio reduction.In other embodiments, drive mechanisms 70 may employ a planetary gearset with a single group of cluster planet gears.

Although not illustrated, front motive members 16 may also be equippedwith final drive mechanisms which are coupled to power source 72 by atransmission. Although less desirable, rear motive members 18 mayalternatively omit final drive mechanisms 70, wherein any speedreduction between power source 72 and rear motive members 18 occursalong transmission 74 and within frame 14.

Steering input device 20 generally comprises a device configured togenerate steering commands which are transmitted to front steeringsubsystem 22 and rear steering subsystem 24 for steering motive members16 and motive members 18. In one particular embodiment, steering inputdevice 20 includes a movable member or shaft 30 which moves in responseto input to generate a steering force which is transmitted along asteering force transmission route 32 to front steering subsystem 22. Thedirection of the force resulting in movement of member 30 generallycorresponds to the direction in which motive member 16 (and possiblymotive member 18) are to be turned. The distance or angle by whichmember 30 is moved generally corresponds to the desired angulardisplacement of motive member 16 (and possibly that of motive member18). In the particular embodiments, the exact angular displacement ofmotive member 16 or 18 may be proportionally increased or decreased. Inone embodiment, steering input device 20 includes a steering wheel whichrotates upon receiving torque from a driver of vehicle 10. The torque istransmitted along force transmission route 32 to subsystem 22. Forcetransmission route 32 continuously transmits the force (torque) fromdevice 20 to subsystem 22.

In one embodiment, force transmission route 32 comprises one or moremechanical links or shafts coupled between device 20 and subsystem 22.Force transmission route 32 may additionally include force augmentingdevices such as hydraulic assist. Force transmission route 32 mayadditionally include ratio adjusting devices configured to augment ordecrement the movement or motion being transmitted along route 32. Inalternative embodiments, force transmission route 32 may includehydraulic lines for transmitting force between device 20 and subsystem22. Although less desirable, force transmission route 32 may be omittedin favor of an electronic control system which transmits steeringcommands in the form of electronic signals from device 20 to subsystem22.

In lieu of including a steering wheel, input device 20 may alternativelyinclude other means for inputting force for generating steering commandssuch as linearly movable input devices of the type commonly employed onskid steering vehicles. Although less desirable, steering input device20 may alternatively include an electronic control system which, inresponse to electronically generated steering command or steeringcommands generated by the driver manually entering steering informationsuch as by the depressment of buttons and the like, moves one or moremovable members 30 to transmit the steering command by force tosubsystem 22 and possibly subsystem 24.

Front steering subsystem 22 generally comprises a system coupled tofront motive members 16 and configured to steer front motive members 16in response to input from device 20. In one embodiment where steeringcommands are transmitted by force from input device 20, subsystem 22includes a movable input shaft 34 which moves as a result of the forcetransmitted from input device 20. Movement of input shaft 34 results inthe steering adjustment of motive member 16. In one embodiment, system22 may comprise a conventionally known rack and pinion front steeringarrangement wherein input shaft 34 is coupled to a pinion gear thattranslates a rack that is coupled to tie rods coupled to motive members16. In an alternative embodiment, input shaft 34 comprises an inputshaft coupled to a conventionally known or future developed poweredsteering gear which is coupled to steering arms to steer motive members16. These and various other conventionally known or future developedsystems employed for steering or turning front motive members inresponse to forces transmitted from a steering input may be employed.

As further shown by FIG. 1, subsystem 22 additionally includes an outputshaft 36 which is also coupled to input device 20 so as to rotate inresponse to input from device 20. The force from output shaft 36transmitted along a force transmission route 40 to rear steering controlmechanism 28. Force transmission route 40 includes at least onemechanical link or shaft coupled between output shaft 36 and controlmechanism 28. Although less desirable, force transmission route 40alternatively utilize hydraulic lines for transmitting force betweensubsystem 22 and control mechanism 28.

As shown by FIG. 1, ratio device 26 is disposed along force transmissionroute 40. Ratio device 26 may comprise a mechanism configured to changea ratio of movement along route 40. In one embodiment, device 26comprises a ratio-changing gear box having a 1.5 to 1.0 ratio whereinthe rotation of the output member of device 26 is increased as comparedto the rotation of the input member of device 26 to increase the angulardisplacement into control mechanism 28.

In still other embodiments, force transmission route 40 may additionallyinclude force augmenting devices configured to augment the forcetransmitted from subsystem 22 to control mechanism 28. As will bedescribed hereafter, this force may be necessary or may be beneficial toassist in overcoming the force of biasing the components of mechanism 28from embodiments. In lieu of providing a force augmenting device alongforce transmission route 40, such a force augmenting device may beprovided as part of subsystem 22 or may be provided as part of forcetransmission route 32. Examples of a force transmission augmentingdevice include multipliers and back driving powered slave steeringgears.

Steering control mechanism 28 is included as part of the overall forcetransmission route from input device 20 to rear steering subsystem 24.Mechanism 28 generally includes a movable input member 44 and a movableoutput member 46. The movable input member 44 is coupled to input device20 so as to move in response to input from device 20. As discussedabove, force is transmitted from input device 20 through transmissionroutes 32, through system 22 and through force transmission route 40, toinput member 44 which causes input member 44 to move. Movable outputmember 46 is coupled to rear steering subsystem 24, wherein the rearsteering subsystem 24 adjusts steering of rear motive members 18 inresponse to movement of output member 46.

As schematically shown in FIG. 1, control mechanism 28 operates in twopossible alternative states: (1) a rear steering state 48 and (2) adwell state 50. In the rear steering state 48, force is transmitted frominput member 44 to output member 46 to move the output member. In thedwell state 50, output member 46 does not move in response to movementof input member 44. Since high speed maneuvers only occur within a smallrange of front motive member steering angle, the initiation of rearsteering of motive members 18 is delayed until front motive members 16have been steered beyond small range. The extent to which front motivemembers 16 may be turned from a straight ahead or centered position islarge enough to insure that all high speed maneuvers, such as lanechanges or high speed obstacle avoidance, will occur within this windowor dead-band range. However, at low speeds, the front steering dead-bandis routinely exceeded such that system 12 will steer rear motive members18 in a coordinated ratio with front motive members 16 when rear motivemembers 16 are steered beyond the dead-band range. As a result, steeringmechanism 28 enables rear motive members 18 to be steered for improvedmaneuverability of vehicle 10 at low speeds while preventing steering ofmotive members 18 at high speeds so that high speed stability isequivalent to those vehicles without rear steering.

In the particular embodiment illustrated, system 12 is configured suchthat both front motive members 16 and rear motive members 18 reach theirmaximum steering angles at the same time. In particular, device 26 andmechanism 28 are configured such that maximum steering angles aresimultaneously attained. The maximum steering angles of members 16 and18 need not be equal.

In one particular embodiment of steering system 12, movable input member44 and movable output member 46 of control mechanism 28 have surfacesthat directly engage or contact one another in the rear steering state48 such that force is directly transmitted from member 44 to member 46to move output member 46. In another embodiment, output member 44 andoutput member 46 are indirectly coupled to one another by one or moreintermediate movable physical structures, wherein member 44 directlyengages at least one of the intermediate structures which in turndirectly engages output member 46 to transmit force to output member 46from input member 44 and to move output member 46 in the rear steeringstate 48. In still another alternative embodiment, input member 44 andoutput member 46 are indirectly coupled to one another by a fluid, suchas hydraulic fluid, wherein movement of input member 44 applies force bya piston directly or indirectly coupled to output member 46 so as totransmit force to output member 46 and move output member 46 whencontrol mechanism 28 is in the rear steering state.

In one particular embodiment, rear steering control mechanism 28 isadditionally configured to resiliently bias output member 46 towards acentering position. In response to output member 46 being positioned inthe centering position, rear steering subsystem 24 steers rear motivemembers 18 to a centered position or straight position. As a result,when mechanism 28 is in the dwell state, control mechanism 28automatically repositions output member 46 to the centering position.

Rear steering subsystem 24 is coupled to rear motive members 18 and isconfigured to steer rear motive members 18 in response to movement ofoutput member 46. In the particular embodiment illustrated, rearsteering subsystem 24 includes an input shaft 54 coupled to outputmember 46 and a force transmission route 56. Force transmission route 56is configured to transmit force from output member 46 to input shaft 54so as to move input shaft 54. In one embodiment, force transmissionroute 56 comprises one or more mechanical links or steering shaftsdisposed between member 46 and shaft 54.

In alternative embodiments, other mechanisms may be disposed along forcetransmission route 56. For example, in one embodiment, ratio adjustingdevice 26 may alternatively be disposed between member 46 and shaft 54.In other embodiments, force augmenting devices, such as torquemultipliers may be deployed between member 46 and shaft 54. Althoughless desirable, the transmission of force between member 46 and inputshaft 54 may additionally include hydraulic lines associated withhydraulic motors/pumps for transmitting force from member 46 to shaft54.

Rear steering subsystem 24 includes any one of a variety ofconventionally known or future developed mechanisms configured to steeror turn rear motive members 18 in response to movement of input shaft54. In one embodiment, steering subsystem 24 includes a pinion gearcoupled to input shaft 54 and a rack gear in engagement with the piniongear and coupled to motive members 18 via tie rods and knuckle arms,wherein rotation of input shaft 54 causes the rotation of the piniongear which in turn moves the rack gear to turn motive members 18. In analternative embodiment, steering subsystem 24 includes a hydraulicallypowered steering gear coupled to or including input shaft 54 so as tosteer motive members 18 in response to rotation of input shaft 54. Inanother alternative embodiment, subsystem 24 additionally includes ahydraulic powered steering slave gear coupled to the first masterhydraulic powered steering gear coupled to motive members 18 to provideadditional assistance in steering motive members 18 in ultimate responseto movement of input shaft 54.

FIGS. 2 and 3 illustrate vehicle 110 including steering system 112, onepreferred embodiment of vehicle 10 and system 12 shown in FIG. 1.Steering system 112 is supported by frame 114 and includes front motivemembers 116, rear motive members 118, steering input device 120, frontsteering subsystem 122, rear steering subsystem 124, ratio adjustingdevice 126, and rear steering control mechanism 128. Front motivemembers 116 and rear motive members 118 generally comprise wheelsrotatably coupled to frame 114. Although vehicle 110 is illustrated asincluding two pairs of opposing wheels (rear axles), vehicle 110 andsteering system 112 may alternatively include a single rear axle orgreater than two rear axles or multiple front axles.

Steering input device 120 generally includes a steering shaft 162coupled to a column and ultimately to a steering wheel (not shown).Steering column 162 is further coupled to front steering subsystem 22.Rotation of the steering wheel results in torque being transmitted tofront steering subsystem 122.

Front steering subsystem 122 generally includes hydraulic poweredsteering gear 164, hydraulic powered back driving slave gear 166, Pitmanarms 168, 169, tie rod 170, steering links 172, 173 and steering arms174, 175. Steering gear 164 receives torque from device 120 androtatably drives Pitman arm 168. As torque is applied to input shaft ofgear 164 which is coupled to a ball screw/piston which is in turncoupled to an output shaft, a valve actuates and forces hydraulic fluidagainst the piston, with the piston rotating the output shaft. Steeringgear 164 is further hydraulically coupled to back driving powered slavesteering gear 166 via hydraulic lines (not shown). In the particularembodiment illustrated, powered steering gear 164 generally comprisesM-series steering gear, Model No. M100, sold by R.H. Sheppard Company,101 Philadelphia Street, Post Office Box 877, Hanover, Pa. 17331.

Back driving powered slave steering gear 166 is coupled to poweredmaster steering gear 164. Steering gear 166 receives pressurizedhydraulic fluid from steering gear 164 to drive its piston which causesrotation of its output shaft. Steering gear 166 rotatably drives Pitmanarm 169. In the particular embodiment illustrated, gear 166 comprises aconventionally known back-driving slave gear, Model No. M90, sold byR.H. Sheppard Company.

Pitman arms 168 and 169 are linked by tie rod 170 so as to move with oneanother. Pitman arms 168 and 169 are pivotally coupled to steering links172 and 173 which are pivotally coupled to steering arms 174 and 175.Steering arms 174 and 175 are rigidly coupled or affixed to steeringknuckles of motive members 116. Rotation of Pitman arms 168 and 169applies linear force to steering arms 174 and 175 to turn front motivemembers 116.

As best shown by FIG. 2, system 112 includes a force transmission route140 including steering shafts 176 which serve as mechanical links fromthe output shaft of gear 166 to input member 144 and control mechanism128. Disposed within force transmission route 140 is a ratio device 126which generally comprises gear box having a first sized gear coupled tosteering shafts 176, and a second sized gear coupled to steering shafts177. In the particular embodiment illustrated, ratio adjusting device126 has a steering ratio of approximately 1.5 to 1.0. As a result, forevery 1.0 revolution of steering shafts 176, steering shafts 177 willrotate 1.5 revolutions.

Rear steering control mechanism 128 (which will be described in greaterdetail hereafter with respect to FIGS. 4-7) has an input member 144 andan output member 146. Input member 144 is rotatably driven uponreceiving torque from steering shaft 177. In a rear steering state, theforces transmitted from input member 144 to output member 146 rotateoutput member 146. In a dwell state, output member 146 does not rotatein response to rotation of input member 144.

While in the dwell state, control mechanism 128 additionally biasesoutput member 146 towards a predetermined position. In particular,mechanism 128 biases output member 146 towards a centering positionwhich causes motive members 118 to be steered to a straight position.

As best shown by FIGS. 2 and 3, system 112 includes a force transmissionroute 156 coupled between output member 146 and input shaft 154 of rearsteering subsystem 124. Transmission route 156 includes a plurality ofsteering shafts 180 which provide a mechanical link between outputmember 146 and input shaft 154.

Rear steering subsystem 124 moves rear motive members 118 in response torotation of output member 146. In the particular embodiment illustrated,rear steering subsystem 124 steers output members 118 in response torotation of input shaft 154. Rear steering subsystem 124 generallyincludes hydraulic powered master steering gear 184, hydraulic poweredslave steering gear 186, double-ended Pitman arms 188, 189, tie rods190, steering links 192, 193, 194, 195, and steering arms 196, 197, 198,and 199. Hydraulic powered steering gear 184 is substantially identicalto hydraulic powered steering gear 164 except that instead of receivingtorque from steering column 162 of steering input device 120, hydraulicpowered steering gear 184 receives torque from one of steering shafts180. As conventionally known, input shaft 154 is coupled to mating ballscrew nut or piston which drives an output shaft. As torque is appliedto the input shaft, a valve actuates hydraulic fluid against the pistonto further assist in rotating the output shaft. The rotation of theoutput shaft helps in the pivoting of double acting Pitman arm 188 (bestshown in FIG. 2).

Hydraulic powered slave steering gear 186 is substantially identical tohydraulic powered slave steering gear 166 except that steering gear 186does not include a back driving output shaft. Steering gear 196 receivespressurized hydraulic fluid from steering gear 194 which serves as amaster. The hydraulic fluid supplied to steering gear 196 is based uponthe torque inputted to input shaft 154. The hydraulic fluid supplied tosteering gear 186 drives a piston which drives an output shaft coupledto double acting Pitman arm 189.

Tie rods 190 are pivotally coupled to and between Pitman arms 188 and189. Pitman arms 188 and 189 are driven in conjunction with one anotherby steering gears 184 and 186. Pitman arms 188 and 189 are pivotallycoupled to steering links 192, 193, 194 and 195, which are pivotallycoupled to steering arms 196, 197, 198 and 199. Steering arms 196, 197,198 and 199 are rigidly coupled or affixed to steering knuckles (notshown) of motive members 118. The rotation of Pitman arms 188 and 189linearly moves links 192, 193, 194 and 195 to pivot steering arms 196,197, 198 and 199, respectively, so as to steer or turn rear motivemembers 118.

FIGS. 4-8 illustrate rear steering control mechanism 128 in greaterdetail. As best shown by FIG. 4, control mechanism 128 is generally aself-contained unit including housing assembly 210, bearings 212, 214,seals 216, 218, input member 144, output member 146, and centeringmechanism 220. Housing assembly 210 generally comprises a structureconfigured to surround and substantially enclose the remaining membersof mechanism 128, as well as to rotatably support input member 144 andoutput member 146. Housing 222 additionally includes mounting pads (notshown) to mount mechanism 128 to a vehicle. In the particular embodimentillustrated, housing assembly 210 includes a main housing 222 and twoend covers 223, 224 secured to main housing 222. In alternativeembodiments, housing assembly 210 may have any one of a variety ofalternative configurations with fewer or greater sections thatcollectively surround and support the remaining components of mechanism128.

Bearings 212 and 214 rotatably support input member 144 and outputmember 146, respectively, within housing 210. Bearing 212 is situatedbetween cover 223 and input member 144. Bearings 214 are situatedbetween cover 224 and output member 146.

Seals 216 and 218 seal about input member 144 and output member 146,respectively, to form a sealed enclosure about dwell mechanism 228 ofinput member 144 and output member 146. As a result, lubricating fluidmay be supplied to the interior of housing 210. In the particularembodiment illustrated, the interior of housing 210 will beapproximately half full of lubricating fluid or oil if mountedhorizontally. In angled installations, housing 210 will be filled to agreater extent so that bearings 212 and 216 operate in lubricating oil.Housing 210 additionally includes a vent port 230 in which a breather(not shown) is positioned. Alternatively, a remote vent line may beused.

Bearings 214 preferably comprise tapered roller bearings so as to retainoutput member 146 in place. Bearings 214 are preferably set with aslight amount of preload to minimize axial end play. Bearings 214 areattached to output member 146 and cover 224 with snap rings 226 andcollapsible spacers 227. Bearings 212 extend about input member 144 witha minimal clearance fit. In alternative embodiments, bearings 214 arealternatively retained in place by conventionally known jam nuts.

FIGS. 5 and 6 illustrate input member 144 and output member 146 ingreater detail. As shown by FIGS. 5 and 6, input member 144 generallyincludes an axially projecting tang 240 which includes surfaces 242 and244. Output member 146 includes a radially projecting shear pin 248which is preferably sized to act as a shear device to prevent excessivetorque between input member 144 and output member 146. Shear pin 248includes surfaces 250 and 252. Surfaces 242 and 244 are angularly spacedfrom one another by angle θ to create a dead-band or dwell range 254comprising the angular extent to which input member 144 may be rotatedabout axis 245 without surface 242 engaging surface 250 or withoutsurface 244 engaging surface 252. The total dwell range 254 is generallyequal to 360 degrees−θ−α (the angular width separating surfaces 250 and252).

As shown by FIG. 6, each of surfaces 242 and 244 is angularly offsetfrom axis 245 by a distance D (the spacing between one of surfaces 242,244 and a reference line parallel to one of surfaces 242, 244 andintersecting axis 245) equal to one half of the diameter of shear pin248. As a result, surfaces 250 and 252 of shear pin 248 contact surfaces242 and 244, respectively, substantially along the entire length of suchsurfaces. This line contact reduces wear of shear pin 248 and tang 240and ensures more reliable dwell ranges. In the particular embodimentshown, the offset distance D is approximately 0.149 inches while pin 248has a diameter of approximately 0.298 inches. Although less desirable,surfaces 242 and 244 may alternatively be configured to omit such anoffset.

FIGS. 5 and 6 illustrate control mechanism 128 in the dwell state inwhich surfaces 242 and 244 are out of engagement with surfaces 250 and252, respectively. As a result, output member 146 does not move inresponse to movement of input member 144. FIGS. 5 and 6 furtherillustrate output member 146 in a centering position which causes rearmotive members 118 (shown in FIGS. 2 and 3) to be steered to a straightposition. FIGS. 5 and 6 further illustrate input member 144 in acentered position, wherein front motive members 116 are also steered toa straight position. In the particular embodiment illustrated, surfaces242 and 244 of input member 144 are equi-angularly spaced from surfaces250 and 252, respectively, when input member 144 is in the centeredposition and when output member 146 is in its centering position. As aresult, input member 144 may be rotated in the direction indicated byarrow 258 by β degrees or the direction indicated by arrow 260 by βdegrees. In the particular embodiment illustrated, the angle θ isapproximately 40 degrees and a is approximately 35 degrees, and angle βis approximately 142.5 degrees. In alternative embodiments, however, theangles θ, α and β may be varied depending upon the desired dwell range.Moreover, in alternative embodiments, input member 144 and output member146 may be angularly repositioned relative to one another such thatsurfaces 242 and 244 are not equi-angularly spaced from surfaces 250 and252, respectively, when input member 144 is in a centered position orwhen output member 146 is in a centering position. For example, inparticular applications, it may be beneficial to have a shorter dwellrange portion between surfaces 242 and 250 as compared to the dwellrange portion between surfaces 244 and 252.

Once input member 144 has been rotatably driven about an angle greaterthan β in the direction indicated by arrow 258 and moved through theassociated distance, surface 244 will contact surface 252 so as totransmit torque or force from input member 144 to output member 146.This transmitted force will result in output member 146 also beingrotatably driven in the direction indicated by arrow 258 so as to causethe rotation of rear motive members 118 in a first direction. Likewise,once input member 144 has been rotatably driven about an angle greaterthan beta in the direction indicated by arrow 260, surface 242 willengage surface 250 so as to transmit force to output member 146 so as torotatably drive output member 146 about axis 245 in the directionindicated by arrow 260. This resulting rotation of output member 146will cause rear motive members 118 to be steered in a second oppositedirection.

Although surfaces 242 and 244 are illustrated as being provided by atang integrally formed as part of a single unitary body as part of inputmember 144, surfaces 242 and 244 may alternatively be provided by avariety of other shapes or configurations integrally formed as part ofinput member 144 or directly or indirectly attached to input member 144.Although surfaces 250 and 252 are illustrated as being provided by ashear pin mounted to output member 146, surfaces 250 and 252 mayalternatively be provided by other structures having differingconfigurations which are integrally formed as part of a single unitarybody with output member 146 or that are directly or indirectly attachedto output member 146.

As best shown by FIG. 4, mechanism 128 additionally includes analignment guide 270 configured to assist in the alignment of inputmember 144 and output member 146 relative to one another duringassembly. In particular, as shown in FIG. 4, input member 144 includes abore 272 which receives an end portion of output member 146. The bore272 has an internal surface 274 forming a depression or detent 276. Theaxial end portion of output member 146 includes a bore 278 receiving aspring 280 and a detent-engaging member 282 (shown as a ball). Spring280 resiliently urges detent-engaging member 282 into engagement withdetent 276 to assist in proper angular alignment of members 144 and 146.

In alternative embodiments, alignment guide 270 may be provided with avariety of other configurations. For example, in lieu of member 144receiving member 146, this relationship may be reversed. Furthermore, inlieu of member 144 including a detent, while member 146 includes aresiliently biased detent-engaging member, member 144 may include aresiliently biased detent-engaging member, while member 146 includes adetent.

Centering mechanism 220 generally comprises a mechanism configured toresiliently bias output member 146 to the centering position. Centeringmechanism 220 generally includes nut 302, nut guide 303, carriage 304,stationary surface 306, stationary surface 308, drive structure 309,spring 310 and end play adjuster 312. Nut 302 generally comprises astructure threadably coupled to output member 146 and guided to movealong axis 314 of output member 146 in conjunction with the rotation ofoutput member 146. In the particular embodiment illustrated, nut 302comprises a ball nut wherein output member 146 includes a ball screwportion extending through nut 302. In alternative embodiments, othertypes of nuts and screw arrangements may be employed.

Nut guide 303 generally comprises a structure configured to engage nut302 so as to substantially prevent rotation of nut 302 while permittingnut 302 to move along axis 314 in conjunction with rotation of outputmember 146. In the particular embodiment illustrated, nut guide 303comprises a tapered pin and jam nut passing through main housing 222into engagement with nut 302. Although less desirable, in alternativeembodiments, nut guide 303 may comprise other structures integrallyformed as part of a single unitary body with housing 222 or otherstructures mounted to housing 222 which serves the same function.

Carriage 304 cooperates with spring 310 to resiliently bias nut 302towards a predetermined position along axis 314 so as to alsoresiliently bias output member 146 towards the centering position.Carriage 304 generally includes drive face 320 and drive face 322. Driveface 320 faces a first side of nut 302 and is axially fixed relative todrive face 322 which faces a second side of nut 302. In the particularembodiment illustrated, drive face 320 is provided by a drive plate 324while drive face 322 is provided by a guide plate 326. Plates 324 and326 are axially fixed relative to one another by three tension rods 328(only one of which is shown). Guide plates 324 and 326 have minimalclearance relative to the inside diameter of housing 222 to assist inmaintaining alignment of spring 310 and rods 328. Tension rods 328 aremounted between plates 324 and 326 and generally pass through nut 302and spring 310 between input member 144/output member 146 and mainhousing 222. Nut 302 is precisely guided by the housing 222. The rods328 are only guided rotationally about axis 314 by nut 302. The rods areguided by plates 326 and 324. There is clearance between rods and matingholes in nut 302.

Although less desirable, in alternative embodiments, carriage 304 mayinclude greater or fewer tension rods. Such tension rods need notnecessarily pass through nut 302. In still other alternativeembodiments, faces 320 and 322 may be provided by other structures otherthan plates and may be fixed relative to one another by other membersmounted between such drive faces or other portions integrally formed aspart of a single unitary body with one or both of the structuresproviding faces 320 and 322.

Spring 310 comprises preloaded centering spring which acts throughoutput member 146 to provide torque to rear steering gear 184 to holdinput shaft 154 of rear steering subsystem 124 in a centered positionwherein rear motive members 118 are retained in a centered position.Since rear steering subsystem 124 is hydraulically powered, the amountof torque required to maintain gears 184 and 186 in a centered positionis relatively low (less then 50 in.-lbs.). As a result, when the driverdecides to straighten out rear motive members 118 from a steeredposition, the driver actuates steering input device 120 to rotate inputshaft 144 in a direction allowing spring 310 to apply the necessarytorque to bring rear steering subsystem 124 to a centering position. Nodriver's effort is required to return rear motive members 118 to thestraight ahead position.

Spring 310 is captured between drive face 320 and drive structure 309.Spring 310 biases drive structure 309 against nut 302 and stationarysurface 306. Spring 310 also biases drive surface 322 against nut 302and stationary surface 308 so as to bias nut 302 and output member 146to a centering position. In addition, spring 310 resiliently biasesoutput member 146 against rotation.

In the particular embodiment illustrated, stationary surfaces 306 and308 are formed along the inner surface of housing 222 as part of asingle unitary body with housing 222. In alternative embodiments,stationary surfaces 306 and 308 may be provided by a single structure ormultiple structures which are mounted or otherwise attached to housing222.

Drive structure 309 generally comprises a structure slidably supportedwithin housing 222 for movement along axis 314 and positioned betweenspring 310 and nut 302. Drive structure 309 is configured to abutstationary surface 306 such that the movement of drive structure 309towards cover 224 is limited. Drive structure 309 acts as an interfacebetween nut 302 and spring 310. In the particular embodimentillustrated, drive structure 309 does not perform any guiding functionand its outer diameter does not contact housing 222.

End play adjuster 312 generally comprises a device configured tominimize or eliminate axial end play of nut 302 to prevent rotary playof output member 146 and to axially maintain straight ahead steeringalignment of rear motive members 18. In the particular embodimentsillustrated, adjuster 312 is provided by three set screws and jam nuts(only one set of which is shown) which thread into and through plate 326into engagement with nut 302 to take up any clearance between nut 302and surfaces 306 and 308. In alternative embodiments, the three setscrews and jam nuts may alternatively be provided on drive structure309. In lieu of comprising set screws and jam nuts, adjustment adjuster312 may utilize a variety of other conventionally known or futuredeveloped structures configured for taking up tolerance between nut 302and surfaces 306, 308.

FIGS. 7 and 8 illustrate steering mechanism 128 in the rear steeringstate in which input member 144 has transmitted torque to output member146 to rotate output member 146 out of its centering position againstthe bias of spring 310. FIG. 7 illustrates the rotation of output member146 in a first clockwise direction, while FIG. 8 illustrates rotation ofoutput member 146 in a second counterclockwise direction. FIG. 7illustrates input member 144 rotated so as to rotatably drive outputmember 146 in a clockwise direction. As a result of the rotation ofoutput member 146, nut 302 has been moved along axis 314 towards cover224. During such movement, nut 302 directly or indirectly engages driveface 322, depending upon whether adjuster 312 is in play, to movecarriage 304 along axis 314 towards cover 224. Consequently, drive face320 is also moved towards cover 224 while compressing spring 310 againstdrive structure 309.

FIG. 8 illustrates rotation of input member 144 and output member 146 ina counterclockwise direction which causes nut 302 to move along axis 314in a direction towards cover 223. As a result, nut 302 engages drivestructure 309 and moves drive structure 309 along axis 314 to compressspring 310 against drive face 320. As shown by FIGS. 7 and 8, centeringmechanism 220 resiliently biases output member 146 towards the centeringposition shown in FIG. 4. This is accomplished utilizing a single spring310.

FIGS. 9-13 illustrate alternative embodiments of centering mechanism220. FIGS. 9 and 10 schematically illustrate rear steering controlmechanism 428, an alternate embodiment of control mechanism 128. Controlmechanism 428 is substantially similar to control mechanism 128 exceptthat mechanism 428 includes nut guide 403 in lieu of nut guide 303. Nutguide 403 generally consists of at least one semi-cylindrical groove 410formed within housing 222 and an opposite semi-cylindrical groove 412formed within nut 302. Grooves 410 and 412 cooperate to receive one ofrods 328. As shown by FIG. 9, rods 328 extend about a perimeter ofspring 310 rather than through spring 310. Rods 328 guide movement ofnut 302 along axis 314 in conjunction with the rotation of output member146 while preventing rotation of nut 302. The remaining functions ofmechanism 428 are substantially similar to mechanism 128.

FIG. 11 illustrates rear steering control mechanism 528, a secondalternative embodiment of steering system 128. Steering mechanism 528 issubstantially similar to steering mechanism 128 except that steeringmechanism 528 utilizes an alternative carriage 504 in which drive faces320 and 322 are provided as part of an elongate unitary sleeve extendingabout nut 302. Although not illustrated, portion 506 of carriage 504comprising a sleeve that extends about nut 302 is preferably keyed tonut 302 to enable nut 302 to move along axis 314 without rotating inconjunction with the rotation of output member 146. The internalconfiguration of housing 222 engages and guides movement of carriage 504along axis 314 while preventing rotation of carriage 504.

FIG. 12 schematically illustrates rear steering control mechanism 628, athird alternative embodiment of mechanism 128. Mechanism 628 is similarto mechanism 128 except that mechanism 628 includes nut 602 in lieu ofnut 302 and spring compression members 604, 606 in lieu of carriage 304.Nut 602 is similar to nut 302 in that nut 602 is threadably coupled tooutput member 146. Like nut 302, nut 602 preferably comprises a ballscrew nut threadably engaging the ball screw portions of output member146. Nut 602 includes end portions 608 and 610 which have faces 612 and614 that face one another and that engage compression members 604 and606. Portions 608 and 610 are preferably keyed to housing 222 to preventmovement of nut 602 along axis 314 while rotation of nut 602.

Alternatively, mechanism 628 may include a nut guide 303 passing throughhousing 222 into engagement with either portion 608 or 610. Althoughportions 608 and 610 are illustrated as being mounted to the remainderof nut 602, portions 608 and 610 may alternatively be integrally formedas part of a single unitary body with nut 602. In still anotheralternative embodiment, nut 602 may be keyed with one or both ofcompression members 604 and 606 wherein one or both of compressionmembers 604 and 606 are also keyed with respect to housing 222 to enablemovement of nut 602 along axis 314 without rotation of nut 602.

Compression members 604 and 606 extend on opposite axial end portions ofspring 310 and at least partially circumscribe nut 602. Compressionmember 604 includes an end portion 618 captured between spring 310 andface 612. Similarly, compression member 606 includes an end portion 620captured between spring 310 and face 614 of nut 602. Movement of nut 602along axis 314 out of a centering position causes one of faces 612 or614 (depending upon the direction of movement of nut 602) to engage oneof compression members 604 or 606 so as to compress spring 310. Onceinput member 144 is rotated out of force transmitting engagement withoutput member 146, spring 310 acts upon one of members 604, 606 toreturn nut 602 to the centering position, whereby output member 146 isalso rotated to the centering position.

FIG. 13 schematically illustrates rear steering control mechanism 728, afourth alternative embodiment of mechanism 128. Steering mechanism 728is similar to steering mechanism 128 except that steering mechanism 728includes centering mechanism 780 in lieu of centering mechanism 220.Centering mechanism 780 includes nut 702, backing plates 705, 707, endplates 709, 711, and compression springs 713, 715. Nut 702 issubstantially identical to nut 302. Nut 702 is threadably coupled tooutput member 144 so as to axially move along axis 314 in conjunctionwith the rotation of output member 144. Although not shown, mechanism728 additionally includes a nut guide substantially identical to nutguide 303 in FIG. 4, wherein the nut guide passes through a portion ofhousing 222 and is in engagement with nut 702 so as to prevent rotationof nut 702 while permitting movement of nut 702 along axis 314.

In operation, the rotation of output member 144 away from a centeringposition results in movement of nut 702 either to the left or to theright along axis 314 depending upon the direction in which output member144 is rotated. For example, movement of nut 702 to the left will causenut 702 to move backing plate 705 along axis 314 towards end plate 709to compress spring 713. Similarly, movement of nut 702 to the right willresult in backing plate 707 being moved along axis 314 to compressspring 715 against end plate 711. Rotation of input member 146 out offorce transmitting engagement with input member 144 enables one ofsprings 713, 715 to resiliently return nut 702 to the centering positionshown in FIG. 13 which results in output member 144 also being rotatedto the centering position.

In the particular embodiment illustrated, end plates 709 and 711 areformed as discrete plates stationarily supported within housing 222through which output member 144 extends. Although not shown, housing 222also preferably surrounds and encloses dwell mechanism 228 between inputmember 146 and output member 144. Although less desirable, inalternative embodiments, end plates 709 and 711 may be provided as endcovers of housing 222.

FIG. 14 schematically illustrates rear steering control mechanism 828, afifth alternative embodiment of mechanism 128. Mechanism 828 includesinput member 844, output member 846, and centering mechanism 920. Inputmember 844 and output member 846 are joined by a dwell mechanism 928.Dwell mechanism 928 includes an engagement arm 940 having opposingsurfaces 942, 944 and output portion 946 having a slot 948 definingsurfaces 950 and 952. Arm 940 is coupled to or integrally formed as partof input member 844 so as to rotate with input member 844. Input member844 is analogous to input member 144 of system 128. Output portion 946is coupled to or integrally formed as part of output member 846 so as torotate with output member 846. Arm 940 projects through slot 948 suchthat sufficient rotation of input member 844 results in either surface942 engaging surface 950 or surface 944 engaging surface 952. In therear steering state, one of surfaces 942, 944 is in engagement with oneof surfaces 950, 952, respectively, to transmit force from input member844 to output member 846 and so as to rotate output member 846. In thedwell state, surfaces 942 and 944 are both out of engagement withsurfaces 950 and 952. Although slot 948 is illustrated as angularlyextending about axis 914 by approximately 160 degrees, the extent towhich slot 948 extends about axis 914 may be modified depending upon thedesired dead-band range of dwell mechanism 928.

Centering mechanism 920 generally includes spring 970 and impact shafts972, 974, 976 and 978. Spring 970 generally comprises a torsional springhaving opposite end portions 980 and 982. Impact shafts 972 and 974 arestationarily supported by housing 222 or other supporting structures andextend into engagement with end portions 980. Shaft 972 engages endportion 980 on a side of end portion 980 closest to axis 914 while shaft974 engages end portion 982 on a side of end portion 982 distant fromaxis 914. Impact shafts 976 and 978 extend from output member 846 andalso engage end portions 980 and 982 of spring 970. Shaft 976 engagesthe side of end portion 980 closest to axis 914 while shaft 978 engagesend portion 982 on a side of end portion 982 distant from axis 914. As aresult, rotation of output member 846 out of a centering position actsagainst spring 970. Rotation of input member 844 towards the centeringposition shown in FIG. 14 enables torsion spring 970 to act againstshafts 976 and 978 so as to rotate output member 846 to the originalcentering position.

FIG. 15 schematically illustrates vehicle 10 including steering system1012, an alternative embodiment of steering system 112 shown in FIG. 1.Steering system 1012 is substantially identical to steering system 112except that steering system 1012 transmits force from output shaft 30 ofinput device 20 to input member 44 of rear steering control mechanism 28without transmitting such force through front steering subsystem 22. Inone embodiment, steering system 1012 includes a torque splitting device1015 configured to simultaneously transmit torque from output shaft 30to both input shaft 36 of front steering subsystem 22 and to rearsteering control mechanism 28. In one embodiment, device 1015 comprisesa series of gears for dividing torque from output shaft 30. In theparticular embodiment illustrated, device 1015 preferably comprises atee gear box which splits hand wheel output torque from input device 20between front steering subsystem 22 and rear steering subsystem 24.Although devices 1015 and 26 are illustrated as distinct components, inalternative embodiments, torque splitting device 1015 and ratio device26 can be combined. For example, a tee gear box that has a 1:1.5 speedratio between force transmission route 32 and force transmission route40 may be employed (ratio device 26 is then omitted).

As further shown by FIG. 15, steering system 1012 additionally includesforce augmenting device 1017 disposed along force transmission route 40which now runs from device 1015 to control mechanism 28. In theparticular embodiment illustrated, force augmenting device 1017 isconfigured to augment the torque being transmitted along forcetransmission route 40. As a result, augmenting device 1017 augments thetorque being transmitted along route 40 from output shaft 30 to asufficient extent so as to overcome the one or more springs employed inthe centering mechanism of control mechanism 28. In one particularembodiment, force augmenting device 1017 comprises an in-line hydraulictorque multiplier such as the 227 Series Geroler unit sold by EatonCorporation of 14615 Lone Oak Road, Eden Prairie, Minn. 55344. As aresult, device 1017 provides sufficient torque to overcome any centeringspring while not adding significantly to the driver's effort. Inalternative embodiments where mechanism 228 omits a centering mechanismsuch as centering mechanism 220, force augmenting device 1017 may beomitted.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, although different preferredembodiments may have been described as including one or more featuresproviding one or more benefits, it is contemplated that the describedfeatures may be interchanged with one another or alternatively becombined with one another in the described preferred embodiments or inother alternative embodiments. Because the technology of the presentinvention is relatively complex, not all changes in the technology areforeseeable. The present invention described with reference to thepreferred embodiments and set forth in the following claims ismanifestly intended to be as broad as possible. For example, unlessspecifically otherwise noted, the claims reciting a single particularelement also encompass a plurality of such particular elements.

1. A vehicle steering system comprising: a steering input device; afront steering subsystem coupled to front motive members to steer thefront motive members based upon movement of a front steering subsysteminput shaft; a rear steering subsystem coupled to rear motive members tosteer the rear motive members based upon movement of a rear steeringsubsystem input shaft; a first force transmission route from thesteering input device to the front steering subsystem input shaft,wherein force is transmitted from the input device to the front steeringsubsystem input shaft to steer the front motive members; a steeringcontrol mechanism having a movable input member and a movable outputmember, the input member including a first surface, the output memberincluding a second surface, wherein a movable input member is movablethrough a first distance without transmitting force to the output memberand is movable through a second distance in which force is transmittedto the output member to move the output member, wherein the firstsurface and the second surface engage one another while the input memberis in the second distance and wherein the first surface and the secondsurface are out of engagement while the input member is in the firstdistance; a second force transmission route from the steering inputdevice to the input member, wherein force is transmitted from the inputdevice to the input member to move the input member; and a third forcetransmission route from the output member to the rear steering subsysteminput shaft to move the rear steering subsystem input shaft and to steerthe rear motive members.
 2. The system of claim 1, wherein the outputmember includes a third surface and a fourth surface, wherein the firstsurface engages the third surface while the input member is in thesecond distance to move the output member in a first direction, whereinthe second surface engages the fourth surface while the input member isin the second distance to move the output member in a second direction,and wherein the first and second surfaces are out of engagement with thethird and fourth surfaces, respectively, while the input member is inthe first distance.
 3. The system of claim 1, wherein the input memberrotates about an axis and wherein the first and second surfaces areangularly spaced about the axis to form a dead band angle about the axisin which the input member may rotate while in the first distance.
 4. Thesystem of claim 3, wherein the dead band angle is approximately 285degrees.
 5. The system of claim 1, wherein the rear motive members arebiased towards a predetermined position while the input member is in thefirst distance.
 6. The system of claim 5, wherein the rear motivemembers are biased towards a straight position while the input member isin the first distance.
 7. The system of claim 1, wherein the outputmember is biased towards a centering position and wherein the rearsteering subsystem steers the rear motive members to a straight positionin response to the output member being in the centering position.
 8. Thesystem of claim 7, wherein the control mechanism includes a springcoupled to the output member to bias the output member to the centeringposition.
 9. The system of claim 1, wherein the second forcetransmission route includes a force augmenting device.
 10. The system ofclaim 9, wherein the input member is rotated to generate a first torqueand wherein the force augmenting device augments the first torque to asecond larger torque.
 11. The system of claim 10, wherein the augmentingdevice includes at least one hydraulic torque multiplier.
 12. The systemof claim 1, further comprising a force splitting device separating thefirst force transmission route from the second force transmission route.13. The system of claim 12, wherein the force splitting device comprisesa series of gears for dividing force from the steering input device. 14.The system of claim 13, wherein the force splitting devicesimultaneously transmits force along the first force transmission routeand the second force transmission route.
 15. A vehicle steering systemcomprising: a steering input device; a torque splitting device coupledto the steering input device and having a first output and a secondoutput; a front steering subsystem coupled to the first output of thetorque splitting device and configured to adjust steering of frontvehicle motive members; and a rear steering control mechanism including:a movable input member coupled to the second output of the torquesplitting device and having a first surface; and a movable output membercoupled to a rear steering subsystem and having a second surface,wherein the rear steering subsystem is configured to adjust steering ofrear vehicle motive members, wherein the control mechanism operates in arear steering state in which force is transmitted from the input memberto the output member to move the output member and a dwell state inwhich the output member does not move in response to movement of theinput member, wherein the first surface and the second surface engageone another while the control mechanism operates in the rear steeringstate and wherein the first surface and the second surface are out ofengagement while the control mechanism operates in the dwell state. 16.The system of claim 15, further comprising a force transmission routefrom the torque splitting device to the input member, wherein the forcetransmission route includes a force augmenting device.
 17. The system ofclaim 16, wherein the second output of the torque splitting deviceprovides a first torque and wherein the force augmenting device augmentsthe first torque to a second larger torque.
 18. The system of claim 17,wherein the force augmenting device is a hydraulic torque multiplier.19. The system of claim 15, farther comprising a ratio device operablycoupled between the input device and the control mechanism.
 20. Thesystem of claim 19, wherein the ratio device is a separate componentfrom the torque splitting device.
 21. A method for controlling a frontsteering subsystem to steer front motive members and a rear steeringsubsystem to steer rear motive members on a vehicle, the methodcomprising: applying a first force to a steering input device to move aportion of the device; and simultaneously transmitting a second forceand a third force based on the first force, the second force applied toan input shaft of the front steering subsystem, whereby the frontsteering subsystem adjusts steering of the front motive members based onmovement of the input shaft, the third force applied to an input memberhaving a first surface so as to move the input member through a firstdistance without transmitting force to an output member having a secondsurface and coupled to an input shaft of the rear steering subsystem andthrough a second distance during which the third force is transmittedfrom the input member to the output member to move the output member andthe input shaft of the rear steering subsystem, whereby the rearsteering subsystem adjusts steering of the rear motive members basedupon movement of the output member, wherein the first surface and thesecond surface engage one another while the input member is in thesecond distance and wherein the first surface and the second surface areout of engagement while the input member is in the first distance. 22.The method of claim 21, wherein at least one of the second force and thethird force is greater than the first force.